1. Field of the Invention
The present invention generally relates to an apparatus for generating a high DC voltage to be applied to an X-ray tube, and more particularly, to a high DC voltage generator employing a controller for controlling turn-on/off transition characteristics of a high DC voltage and electric current.
2. Description of the Related Art
High DC voltage-generating apparatuses of this type, for use in an X-ray tube, are known. These conventional high DC voltage generators are disclosed in, for instance, U.S. Pat. Nos. 4,200,795 and 4,573,184.
Referring now to FIGS. 1 and 2, a conventional high DC voltage generator will be described in detail. In the circuit arrangement shown in FIG. 1, three-phase rectifier bridge circuit 1 is connected to receive a low three-phase AC voltage. The DC output of rectifier bridge circuit 1 is supplied to chopper circuit 2. Chopper circuit 2 controls its output voltage. The output of chopper circuit 2 is input to DC-to-AC inverter circuit 4 via filtering circuit 3. The output of inverter circuit 4, i.e., a low AC voltage, is stepped up by high-voltage transformer 5. The high AC voltage boosted by transformer 5 is rectified by high-voltage rectifier bridge circuit 6, smoothed by capacitors C1 and C2, and applied between the anode and filament (i.e., cathode) of X-ray tube 7.
In general, the high DC voltage generator shown in FIG. 1 is designed for use in the X-ray apparatus incorporated in a computerized tomography apparatus. Capacitors C1 and C2 are inserted in the secondary circuit of transformer 5 in order to reduce the ripple components in the high DC voltage (i.e., tube voltage) applied to X-ray tube 7. As is shown in FIG. 2, the turn-on and turn-off transition periods of tube voltage kV become inherently long, due to the existence of the capacitances of capacitors C1 and C2 and, in particular the impedance of the secondary circuit of transformer 5. In FIG. 2, "mA" represents the current flowing through X-ray tube 7 (hereinafter called "tube current"), and "If" denotes the filament current of X-ray tube 7.
Such an X-ray apparatus must be operated not to generate unnecessary X-rays which will not contribute to X-ray image data acquisition, or must be provided with an X-ray shutter for preventing such unnecessary X-rays from penetrating through a patient. Moreover, the X-ray apparatus exhibits a poor repetitive response due to the inherent lengthy turn-on (rising) and turn-off (falling) transition periods, and inevitably emits X-rays which do not contribute to image data acquisition. Further, the X-ray tube is subjected to unwanted thermal input during the rising transition period, and thus cannot operate at a high efficiency.
In summary, when an X-ray tube is driven by the output DC voltage of the conventional high DC voltage-generating apparatus, it exhibits poor repetitive response, emits a large quantity of unnecessary X-rays, and cannot operate at a high efficiency.